Lithium silicate materials

ABSTRACT

Lithium silicate materials are described which can be easily processed by machining to dental products without undue wear of the tools.

This application is a continuation of U.S. patent application Ser. No.16/977,050, filed Aug. 19, 2020, which is a continuation of U.S. patentapplication Ser. No. 16/200,187, filed Nov. 26, 2018, now U.S. Pat. No.11,109,949, which is a continuation of U.S. patent application Ser. No.14/984,490, filed Dec. 30, 2015, now U.S. Pat. No. 10,136,973, which isa continuation of U.S. patent application Ser. No. 13/834,526, filedMar. 15, 2013, now U.S. Pat. No. 9,248,078, which is a divisionapplication of U.S. patent application Ser. No. 12/833,721, filed Jul.9, 2010, now U.S. Pat. No. 8,444,756, which is a continuation-in-partapplication of U.S. patent application Ser. No. 12/509,959, filed Jul.27, 2009, now U.S. Pat. No. 7,816,291, which is a continuation of U.S.patent application Ser. No. 11/935,203, filed Nov. 5, 2007, now U.S.Pat. No. 8,047,021 which is a division of U.S. patent application Ser.No. 10/913,095, filed Aug. 6, 2004, now U.S. Pat. No. 7,316,740, whichclaims priority to German Patent Application No. 103 36 913, filed Aug.7, 2003, and U.S. patent application Ser. No. 12/833,721, filed Jul. 9,2010, now U.S. Pat. No. 8,444,756 is also a continuation-in-partapplication of U.S. patent application Ser. No. 12/253,437, filed Oct.17, 2008, now U.S. Pat. No. 7,871,948, which is a division of U.S.patent application Ser. No. 11/348,053, filed Feb. 6, 2006, now U.S.Pat. No. 7,452,836, which claims priority to European Patent ApplicationNo. EP 05002588, filed Feb. 8, 2005, and German Patent Application No.10 2005 028 637, filed Jun. 20, 2005, all of which are hereinincorporated by reference in their entirety.

This application is related to U.S. patent application Ser. No.12/562,348, filed Sep. 18, 2009, now U.S. Pat. No. 7,955,159, which isherein incorporated by reference in its entirety.

The invention relates to lithium silicate materials which can be easilyshaped by machining and subsequently converted into shaped products withhigh strength.

There is an increasing demand for materials which can be processed intodental restorative products, such as crowns, inlays and bridges, bymeans of computer controlled milling machines. Such CAD/CAM methods arevery attractive as they allow providing the patient quickly with thedesired restoration. A so-called chair-side treatment is thus possiblefor the dentist.

However, materials suitable for processing via computer aideddesign/computer aided machining (CAD/CAM) methods have to meet a veryspecific profile of properties.

First of all, they need to have in the finally prepared restorationappealing optical properties, such as translucence and shade, whichimitate the appearance of the natural teeth. They further need to showhigh strength and chemical durability so that they can take over thefunction of the natural tooth material and maintain these propertiesover a sufficient period of time while being permanently in contact withfluids in the oral cavity which can even be aggressive, such as acidicin nature.

Secondly and very importantly, it should be possible to machine them inan easy manner into the desired shape without undue wear of the toolsand within short times. This property requires a relatively low strengthof the material and is therefore in contrast to the desired propertiesmentioned above for the final restoration.

The difficulty of combining the properties of low strength in the stageof the material to be processed and a high strength of the finalrestoration is reflected by the known materials for a CAD/CAM processingwhich are in particular with respect to an easy machinabilityunsatisfactory.

DE-A-197 50 794 discloses lithium disilicate glass ceramics which areprimarily intended to be shaped to the desired geometry by ahot-pressing process wherein the molten material is pressed in theviscous state. It is also possible for these materials to be shaped bycomputer aided milling processes. However, it has been shown that themachining of these materials results in a very high wear of the toolsand very long processing times. These disadvantages are caused by thehigh strength and toughness primarily imparted to the materials by thelithium disilicate crystalline phase. Moreover, it has been shown thatthe machined restorations show only poor edge strength. The term “edgestrength” refers to the strength of parts of the restoration having onlya small thickness in the range of few 1/10 mm.

Further approaches of achieving easy machinability together with a highstrength of the final restoration have also been made. EP-B-774 993 andEP-B-817 597 describe ceramic materials on the basis of Al₂O₃ or ZrO₂which are machined in an unsintered state which is also referred to as“green state”. Subsequently, the green bodys are sintered to increasethe strength. However, these ceramic materials suffer from a drasticalshrinkage of up to 50% by volume (or up to 30% as linear shrinkage)during the final sintering step. This leads to difficulties in preparingthe restorations with exactly the dimensions as desired. The substantialshrinkage represents a particular problem if complicated restorationsare manufactured, such as a multi-span bridge.

From S. D. Stookey: “Chemical Machining of Photosensitive Glass”, Ind.Eng. Chem., 45, 115-118 (1993) and S. D. Stookey: “PhotosensitivityOpacifiable Glass” U.S. Pat. No. 2,684,911 (1954) it is also known thatin lithium silicate glass ceramics a metastable phase can be formed atfirst. For example in photosensitive glass ceramics (Fotoform®,FotoCeram®) Ag-particles are formed using UV-light. These Ag-particlesserve as crystallization agent in a lithium metasilicate phase. Theareas which were exposed to light are in a subsequent step washed out bydiluted HF. This procedure is possible since the solubility of thelithium metasilicate phase in HF is much higher than the solubility ofthe parent glass. The glass portion remaining after said solubilizingprocess (Fotoform®) can be transferred into a lithium disilicate glassceramic (FotoCeram®) by an additional heat treatment.

Also investigations of Borom, e.g. M.-P. Borom, A. M. Turkalo, R. H.Doremus: “Strength and Microstructure In Lithium DisilicateGlass-Ceramics”, J. Am. Ceream. Soc., 58, No. 9-10, 385-391 (1975) andM.-P. Borom, A. M. Turkalo, R. H. Doremus: “Verfahren zum Herstellen vonGlaskeramiken” DE-A-24 51 121 (1974), show that a lithium disilicateglass ceramic can in the first instance crystallize in varying amountsas metastable lithium metasilicate phase. However, there also existcompositions which crystallize in the form of the disilicate phase fromthe beginning and the metasilicate phase is not present at all. Asystematic investigation of this effect has not become known. From theinvestigations of Borom it is also known that the glass ceramic whichcontains lithium metasilicate as the main phase has a reduced strengthcompared to the one of a glass ceramic which only contains a lithiumdisilicate phase.

U.S. Publication Nos. 2009/0258778, 2009/0256274 and 2010/0083706 toCastillo, which are hereby incorporated by reference, are directed tolithium silicate glass ceramics, comprising lithium metasilicatecrystals formed by heat treatment at 630° to 650° C. for 10 to 40minutes. In Castillo, nucleation and crystal growth are occurringsimultaneously. Nevertheless, the metaslicate block or ingot of Castillowhich is termed pre-nucleated or nucleated, has a strength from 99-145MPa. In Castillo, the dental restoration is made from the intermediateblock or ingot either by CAD/CAM milling and fully crystallizing it from830° to 870° C. or by hot pressing at a temperature from 800° to 870° C.

Thus, the prior art materials show a couple of shortcomings. It is,therefore, an object of the present invention to eliminate thesedisadvantages and in particular to provide a material which, above all,can be easily shaped by computer-aided milling and trimming processesand can subsequently be converted into high-strength dental productswhich also display a high chemical durability and excellent opticalproperties and exhibit a drastically reduced shrinkage during said finalconversion.

This object is achieved by the lithium silicate glass ceramic materialherein described and as set forth in the claims, hereby incorporatedinto the specification.

The invention also relates to a lithium disilicate material, a dentalproduct, processes for the preparation of a lithium silicate blank and adental restoration according to, a lithium silicate glass, a blank, andto methods for manufacturing a lithium silicate restoration or a dentalrestoration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a principle temperature profile of a process according tothe invention starting from the melt via lithium metasilicate to lithiumdisilicate with options (4 a) and (4 b) for nucleation andcrystallization.

FIG. 2 shows a DSC-plot of a lithium silicate material according toexample 13.

FIG. 3 shows a high temperature XRD of a lithium silicate materialaccording to example 13, in form of a bulk glass sample.

FIG. 4 shows an XRD for phase analysis of a lithium silicate materialaccording to example 13 after nucleation and first crystallization.

FIG. 5 shows an SEM-micrograph, back scattered electrons, of a lithiumsilicate material according to example 13 after nucleation and firstcrystallization.

FIG. 6 shows an XRD for phase analysis of a lithium silicate materialaccording to example 13 which was subjected to nucleation, firstcrystallization and second crystallization conditions, and

FIG. 7 shows an SEM-micrograph, back scattered electrons, of a lithiumsilicate material according to example 13 which was subjected tonucleation, first crystallization and second crystallization conditionsand has an etched surface.

It has surprisingly been shown that by using a starting glass of a veryspecific composition and a specific process it is possible to providethe glass ceramic according to the invention which has metastablelithium metasilicate (Li₂SiO₃) as main crystalline phase rather thanlithium disilicate (Li₂Si₂O₅). This lithium metasilicate glass ceramichas a low strength and toughness and hence can be easily machined intothe shape of even complicated dental restorations, but can after suchmachining be converted by a heat treatment into a lithium disilicateglass ceramic product with outstanding mechanical properties, excellentoptical properties and very good chemical stability thereby undergoingonly a very limited shrinkage.

The lithium silicate glass ceramic material according to the inventioncomprises the following components

Component Wt. % SiO₂ 64.0-73.0 Li₂O 13.0-17.0 K₂0 2.0-5.0 A1₂O₃ 0.5-5.0P₂O₅ 2.0-5.0and comprises lithium metasilicate as main crystalline phase.

Another preferred embodiment of the present invention is formed by asilicate glass ceramic material as described above which is formed in aprocess which includes a step wherein lithium metasilicate as maincrystalline phase is produced.

It is preferred that the lithium silicate material of the presentinvention further comprises the following additional componentsindependently from each other

Component Wt. % ZnO 0.5-6.0, preferably 2.0-6.0 Na₂O 0.0-2.0 Me^(II)O0.0-7.0, preferably 0.0-5.0 ZrO₂ 0.0-2.0 colouring and fluorescent0.5-7.5 metal oxideswith Me^(II)O being one or more members selected from the groupconsisting of CaO, BaO, SrO and MgO.

A lithium silicate material which comprises the following components,independently of one another, in the following amounts is particularlypreferred:

Component Wt. % SiO₂ 65.0-70.0 Li₂O 14.0-16.0 K₂O 2.0-5.0 Al₂O₃ 1.0-5.0P₂O₅ 2.0-5.0 ZnO 2.0-6.0 Na₂O 0.1-2.0 Me^(II)O 0.1-7.0, preferably0.1-5.0 ZrO₂ 0.1-2.0 coloring and fluorescent 0.5-3.5 metal oxides

with Me^(II)O being one or more members selected from the groupconsisting of CaO, BaO, SrO and MgO, and

with the metal of the one or more coloring and fluorescent metal oxidesbeing preferably selected from the group consisting of Ta, Tb, Y, La,Er, Pr, Ce, Ti, V, Fe and Mn.

The phrase “. . . independently from each other . . . ” means that atleast one of the preferred amounts is chosen and that it is thereforenot necessary that all components are present in the preferred amounts.

As colouring components or fluorescent components for example oxides off-elements may be used, i.e. the list of metals given above is not to beseen as terminal. The colouring or fluorescent components ensure thatthe colour of the final dental product matches that of the natural toothmaterial of the patient in question.

In the above composition P₂O₅ acts as a nucleation agent for the lithiummetasilicate crystals and a concentration of at least 2 wt % is requiredfor the necessary nucleation. Instead of P₂O₅, other nucleation agentsare also possible, e.g. compounds of the elements Pt, Ag, Cu and W.

In addition to the components mentioned above the glass ceramic mayfurther comprise additional components to enhance the glass technicalprocessability. Such additional components may therefore be inparticular compounds such as B₂O₃ and F which in general amount to 0 to5.0% by weight.

A lithium silicate material as described above is particularly preferredwhich comprises 67.0 to 70.0 wt % of SiO₂.

It has surprisingly been shown that a specific volume portion of lithiummetasilicate should be present to achieve excellent processingproperties. Thus, it is further preferred that the lithium metasilicatecrystalline phase forms 20 to 50 vol % and in particular 30 to 40 vol %of the lithium silicate material. Such a part of the volume leads to thecrystals being present rather remote from each other and hence avoids atoo high strength of the lithium silicate material.

Lithium silicate material of higher crystalinity, i.e., comprisinghigher volume percent of metasilicate phase or combinations ofmetasilicate phase with disilicate phase can be produced for somecompositional ranges by heat treating at higher temperatures such as700° C. (Cycle B) as set forth in U.S. Pat. No. 7,452,836 to Apel etal., compared to 650° C. in 7,316,740 to Schweiger et al. Such materialsare also machinable by larger milling machines, such as DCS Precimill(DCS Switzerland).

The lithium metasilicate crystals are preferably of lamellar or plateletform. This leads to a very good machinability of the lithium silicatematerial without use of high energy and without uncontrolled breaking.The latter aspect of uncontrolled breaking is for example known fromglasses which are generally unsuitable for machining. It is assumed thatthe preferred morphology of the lithium metasilicate crystals is alsoresponsible for the surprisingly high edge strength of products, e.g.complicated dental restorations, can be made from the lithium silicatematerial according to the invention.

The lithium silicate material according to the invention preferably isin the form of a blank. The blank usually takes the form of a smallcylinder or a rectangular block. The exact form depends on the specificapparatus used for the desired computer-aided machining of the blank.

After the machining, the lithium silicate material according to theinvention has preferably the shape of a dental restoration, such as aninlay, an onlay, a bridge, an abutment, a facing, a veneer, a facet, acrown, a partial crown, a framework or a coping.

A lithium disilicate material which is formed in a process whichincludes a step wherein a phase comprising primarily crystalline lithiummetasilicate is produced, the lithium metasilicate being subsequentlyconverted to lithium disilicate forms a preferred embodiment of theinvention.

A dental product made from lithium disilicate, said lithium disilicatebeing formed in a process which includes a step wherein a phasecomprising primarily crystalline lithium metasilicate is produced, thelithium metasilicate being subsequently converted to lithium disilicateforms another preferred embodiment of the present invention.

A blank of lithium silicate glass ceramic material according to theinvention is preferably prepared by a process which comprises

-   -   (a) producing a melt of a starting glass containing the initial        components SiO₂, Li₂O, K₂O, Al₂O₃ and P₂O₅ as the main        components,    -   (b) pouring the melt of the starting glass into a mould to form        a starting glass blank and cooling the glass blank to room        temperature,    -   (c) subjecting the starting glass blank to a first heat        treatment at a first temperature to give a glass product which        contains nuclei suitable for forming lithium metasilicate        crystals,    -   (d) subjecting the glass product of step (c) to a second heat        treatment at a second temperature which is higher than the first        temperature to obtain the lithium silicate blank with lithium        metasilicate crystals as the main crystalline phase.

A process as described above, wherein the starting glass of step (a)further comprises ZnO, Na₂O, Me^(II)O, ZrO₂, and coloring andfluorescent metal oxides, with Me^(II)O being one or more membersselected from the group consisting of CaO, BaO, SrO and MgO ispreferred.

A process as described above, wherein the starting glass of step (a)comprises the following initial components, independently of oneanother, In the following amounts

Component Wt. % SiO₂ 65.0-70.0 Li₂O 14.0-16.0 K₂O 2.0-5.0 Al₂O₃ 1.0-5.0P₂O₅ 2.0-5.0 ZnO 2.0-6.0 Na₂O 0.1-2.0 Me^(II)O 0.1-7.0, preferably0.1-5.0 ZrO₂ 0.1-2.0 coloring and fluorescent 0.5-3.5 metal oxideswith Me^(II)O being one or more members selected from the groupconsisting of CaO, BaO, SrO and MgO and

with the metal(s) of the one or more coloring and fluorescent metaloxides being preferably selected from the group consisting of Ta, Tb, Y,La, Er, Pr, Ce, Ti, V, Fe and Mn is even more preferred.

In step (a), a melt of a starting glass is produced which contains thecomponents of the glass ceramic. For this purpose a correspondingmixture of suitable starting materials, such as carbonates, oxides, andphosphates, is prepared and heated to temperatures of, in particular1300 to 1600° C., for 2 to 10 hours. In order to obtain a particularlyhigh degree of homogeneity, the glass melt obtained may be poured intowater to form glass granules and the glass granules obtained are meltedagain.

In step (b), the melt of the starting glass is poured into acorresponding mould, e.g. a steel mould, and cooled to room temperatureto give a glass product.

The cooling is preferably conducted in a controlled manner so as toallow a relaxation of the glass and to prevent stresses in the structureassociated with rapid temperature changes. As a rule, the melt istherefore poured into preheated moulds, e.g. of a temperature of about400° C. Subsequently, the product can slowly be cooled in a furnace toroom temperature.

In step (c) the starting glass product is subjected to a first heattreatment at a first temperature to cause formation of nuclei forlithium metasilicate crystals. Preferably, this first heat treatmentinvolves a heating of the glass product for a period of 5 minutes to 1hour at a first temperature of 450 to 550° C. In some cases it isconvenient to combine step b) and step c) in order to relax the glassarticle and nucleate the lithium metasilicate crystals in one singleheat treatment therefore a process as described above, wherein step (c)is replaced by modifying step (b) such that during the cooling process atemperature of about 450 to 550° C. is held for a period of about 5minutes to 50 minutes to produce the glass product which contains nucleisuitable for formation of the lithium metasilicate crystals during step(b) forms a preferred embodiment of the invention.

A process as described above, wherein in step (c) the first heattreatment comprises heating the starting glass blank to a temperature ofabout 450 to 550° C. for a period of about 5 minutes to 1 hour formsanother preferred embodiment of the invention.

Subsequently, the glass product comprising the desired nuclei is cooledto room temperature.

In the subsequent step (d), the glass product having the desired nucleiof Li₂SiO₃ is subjected to a second heat treatment at a secondtemperature which is higher than the first temperature. This second heattreatment results in the desired formation of lithium metasilicatecrystals as predominant and preferably as only crystalline phase andtherefore gives the lithium metasilicate glass ceramic according to theinvention. Preferably, this second heat treatment of step (d) comprisesheating the glass product which contains nuclei suitable for formationof lithium silicate crystals to a second temperature of about 600 to700° C. for a period of about 10 to 30 minutes.

It has further surprisingly been shown that relatively high temperatureslead to high amounts of lithium metasilicate which in turn lead to ahigh amount of lithium disilicate in the third heat treatment. Such highamounts of lithium disilicate impart a high strength to the ceramic.Thus, if the emphasis is on the achieving a high strength final product,then it is advantageous to carry out the second heat treatment at 680°to 720° C., and preferably 690° to 710° C. and more preferably about700° C.

The principle temperature profile of such a process is exemplified inFIG. 1. Already starting from the melt (1), i.e. at the end of step a)the temperature decreases for relaxation of the product in a temperaturerange of 500 to 450° C. (2). The temperature can then be brought to roomtemperature (solid line), step b), and afterwards be brought to atemperature of about 450 to 550° C. or can be kept in the temperaturerange of 450 to 500° C. (dotted line). In the region that is labeledwith (3), step c) , nucleation occurs at a temperature of 450 to 550° C.and is influenced by P₂O₅. Then, the glass material can be heateddirectly to a temperature in the range of 600 to 700° C. and kept atsaid temperature (4) during which time lithium metasilicate forms, stepd). Subsequently the material can be cooled down (solid line) to e.g.about room temperature for grinding, milling or CAD-CAM processing andcan afterwards be brought to a temperature of about 700 to 950° C. orcan directly be brought to 700 to 950° C. (dotted line) at whichtemperature (5) the second crystallization occurs forming the lithiumdisilicate and where additional heat treatment or hot pressing can beundertaken.

Alternatively, steps 3 and 4 can be combined into one step as shown inFIG. 1 at step 4 a, or alternatively at step 4 b. That is, thenucleation and crystallization may be combined in one step in thetemperature range from room temperature to about 830° C. or higher. Inone embodiment, the one or more heat treatments to convert the glassblanks into machinable glass-ceramic blanks is/are performed in thetemperature range of from about 450° C. to about 830° C.

Depending on the specific composition of a selected starting glass, itis possible for the skilled person by means of differential scanningcalorimetry (DSC) and x-ray diffraction analyses to determine suitableconditions in steps (c) and (d) to result in materials having thedesired morphology and size of the crystals of lithium metasilicate. Tofurther illustrate this process FIGS. 2 to 5 together with Tables I andII in the example section indicate how the relevant data were obtainedfor example 13 using said measurements and are therefore obtainable ingeneral. Moreover, these analyses allow also the identification ofconditions avoiding or limiting the formation of undesirable othercrystalline phases, such as of the high-strength lithium disilicate, orof cristobalite and lithium phosphate.

Subsequent to step (d), it is preferred to shape the obtained glassceramic. This is preferably effected by step (e), wherein the lithiummetasilicate glass ceramic is machined to a glass ceramic product of thedesired shape, in particular the shape of a dental restoration. Themachining is preferably carried out by trimming or milling. It isfurther preferred that the machining is controlled by a computer, inparticular by using CAD/CAM-based milling devices. This allows aso-called chair-side treatment of the patient by the dentist.

It is a particular advantage of the glass ceramic according to theinvention that it can be shaped by machining without the undue wear ofthe tools observed with the tough and high-strength prior art materials.This is in particular shown by the easy possibility to polish and trimthe glass ceramics according to the invention. Such polishing andtrimming processes therefore require less energy and less time toprepare an acceptable product having the form of even very complicateddental restorations.

Lithium disilicate dental restorations can be produced in many differentways. Commonly used by dental technicians are the CAD/CAM and the hotpressing technique. Dentists can use a CAD/CAM method (Cerec 2®, Cerec3®) to produce chair-side an all ceramic lithium disilicate restoration.The final result is always a dental restoration with lithium disilicateas the main crystalline phase. For this purpose, the blank can be alithium metasilicate glass ceramic according to the invention. The glassceramic according to the invention can therefore be processed in bothways, by CAD/CAM or by hot-pressing, which is very advantageous for theuser.

It is also possible to use for these purposes a corresponding lithiumsilicate glass which comprises nuclei suitable for formation of lithiummetasilicate crystals. This glass is a precursor of the lithiummetasilicate glass ceramic of the invention. The invention is alsodirected to such a glass. It is obtainable by the above process in step(c).

For manufacturing a dental restoration by the hot pressing technique alithium silicate glass ingot having nuclei for lithium metasilicate issubjected to a heat treatment of about 700 to 1200° C. to convert itinto a viscous state. The heat treatment will be conducted in a specialfurnace (EP 500®, EP 600®, Ivoclar Vivadent AG). The ingot is embeddedin a special investment material. During the heat treatment the ingotwill be crystallized. The main crystal phase is then lithium disilicate.The viscous glass ceramic flows under a pressure of 1 to 4 MPa into thecavity of the investment material to obtain the desired shape of thedental restoration. After cooling the investment mould to roomtemperature the lithium disilicate restoration can be divested by sandblasting. The framework can be further coated with a glass or a glassceramic by sintering or hot pressing technique to get the finalizeddental restoration with natural aesthetics.

An ingot which comprises the lithium silicate glass ceramic according tothe invention is subjected to a heat treatment of about 700 to 1200° C.to convert it into a viscous state. The heat treatment will be conductedin a special furnace (EP 500®, EP 600®, Ivoclar Vivadent AG). The glassceramic ingot is embedded in a special investment material. During theheat treatment the glass ceramic will be further crystallized. The maincrystal phase is then lithium disilicate. The viscous glass ceramicflows under a pressure of 1 to 4 MPa into the cavity of the investmentmaterial to obtain the desired shape of the dental restoration. Aftercooling the investment mould to room temperature the lithium disilicaterestoration can be divested by sand blasting. The framework can befurther coated with a glass or a glass ceramic by sintering or hotpressing technique to get the finalized dental restoration with naturalaesthetics.

For manufacturing a dental restoration by the CAD/CAM technique thelithium silicate or the lithium metasilicate blocks with lithiumdisilicate as possible minor crystalline phase having a strength ofabout 80 to 150 MPa can be easily machined in a CAM unit like Cerec 2®or Cerec 3® (Sirona, Germany). Larger milling machines such as DCSprecimill® (DCS, Switzerland) are also suitable. The block is thereforepositioned in the grinding chamber by a fixed or integrated holder. TheCAD construction of the dental restoration is done by a scanning processor an optical camera in combination with a software tool. The millingprocess needs for one unit 10 to 15 minutes. Copy milling units such asCelay® (Celay, Switzerland) are also suitable for machining the blocks.First, a 1:1 copy of the desired restoration is fabricated in hard wax.The wax model is then mechanically scanned and 1:1 mechanicallytransmitted to the grinding tool. The grinding process is therefore notcontrolled by a computer. The milled dental restoration has to besubjected to a heat treatment to get the desired lithium disilicateglass ceramic with high strength and tooth like color. The heattreatment is conducted in the range of 700 to 900° C. for a period ofabout 5 to 30 minutes. The framework can be further coated with a glassor a glass ceramic by sintering or hot pressing technique to get thefinalized dental restoration with natural aesthetics.

Blocks stronger than about 180-200 MPa and having upward strengths ashigh as 400 MPa or greater, such as blocks comprising more than about40% lithium metasilicate, lithium disilicate or combinations thereof arepreferably grinded in a large milling machine such as DCS precimill®(DCS,

Switzerland) due to the high strength and toughness of the glassceramic. Grinding such strong blocks in smaller milling machines may beeconomically prohibitive due to long milling times or increased toolwear. The block is therefore positioned in the grinding chamber by afixed metal holder. The CAD construction of the dental restoration isdone by a scanning process in combination with a software tool. Anadditional heat treatment in the range of 700 to 900° C. could beconducted in order to close surface flaws which were induced by thegrinding process. The framework can be further coated with a glass or aglass ceramic by sintering or hot pressing technique to get thefinalized dental restoration with natural aesthetics.

For blocks containing lithium metasilicate, it has been found that evenat high strengths, e.g., about 200 to about 400 MPa range or more, theblocks can be milled on various types of milling machines due to thelower fracture toughness, for example, fracture toughness up to 2.0,preferably up to 1.5. In this way, the milled article or restoration maybe used as is, or may be further heat treated to grow largermetasilicate or crystallize to lithium disilicate.

It has further been shown that the easily machinable lithiummetasilicate glass ceramic according to the invention can be convertedinto a lithium disilicate glass ceramic product by a further heattreatment. The obtained lithium disilicate glass ceramic has not onlyexcellent mechanical properties, such as high strength, but alsodisplays other properties required for a material for dentalrestorations.

Thus, the invention also relates to a process for preparing a lithiummetasilicate glass ceramic, which may be milled as lithium metasilicateand used as is, or further heat treated to grow more or largermetasilicate. The milled restoration is further finished by staining,glazing or veneering with overlay porcelain associated with firing attemperatures within the range of about 350° to about 850° C. Heattreatment to increase crystallinity or convert to disilicate can be doneconcurrently with the latter finishing/firing steps.

The invention also relates to a process for preparing a lithiumdisilicate glass ceramic product, which comprises (f) subjecting thelithium metasilicate glass ceramic according to the invention to a thirdheat treatment to convert lithium metasilicate crystals to lithiumdisilicate crystals.

In this step (f), a conversion of the metastable lithium metasilicatecrystals to lithium disilicate crystals is effected. Preferably, thisthird heat treatment involves a complete conversion into lithiumdisilicate crystals and it is preferably carried out by heating at 700to 950° C. for 5 to 30 minutes. The suitable conditions for a givenglass ceramic can be ascertained by conducting XRD analyses at differenttemperatures.

It was also found out that the conversion to a lithium disilicate glassceramic is associated with only a very small linear shrinkage of onlyabout 0.2 to 0.3%, which is almost negligible in comparison to a linearshrinkage of up to 30% when sintering ceramics.

A process as described above, wherein the lithium silicate blank has abiaxial strength of at least 90 MPa and a fracture toughness of at least0.8 MPam^(0.5) is preferred.

A process as described above, wherein the starting glass blank of step(b), the glass product containing nuclei suitable for forming lithiummetasilicate of step (c), or the lithium silicate blank with lithiummetasilicate as the main crystalline phase of step (d) is shaped to adesired geometry by machining or by hot pressing to form a shapedlithium silicate product is also preferred.

Such a process, wherein the shaped lithium silicate blank is a dentalrestoration is more preferred and a process wherein the dentalrestoration is an inlay, an onlay, a bridge, an abutment, a facing, aveneer, a facet, a crown, a partial crown, a framework or a coping iseven more preferred.

A process as described above, wherein the machining is performed bygrinding or milling forms a preferred embodiment of the invention,whereby a process wherein the machining is controlled by a computer iseven more preferred.

A process as described above but further comprising subjecting theshaped lithium silicate product to a third heat treatment at a thirdtemperature of about 700 to 950° C. for a period of about 5 to 30minutes is another aspect of the present invention and said process isparticularly preferred when the lithium silicate product subjected tothe third heat treatment comprises lithium metasilicate as the maincrystalline phase, and wherein the third heat treatment converts thelithium metasilicate crystals to lithium disilicate crystals as the maincrystalline phase of the dental restoration.

A process as described above wherein the lithium silicate productsubjected to the third heat treatment comprises the glass productcontaining nuclei suitable for forming lithium metasilicate crystals,and wherein lithium disilicate crystals are crystallized directly fromthe nuclei suitable for forming lithium metasilicate crystals is alsopreferred.

Another preferred embodiment of the present invention is a process asdescribed above, wherein the shrinkage that occurs during the third heattreatment is less than 0.5% , preferably less than 0.3%, by volume.

A process as described above which comprises shaping of a lithiumsilicate material to the desired geometry by hot pressing to produce thedental restoration is also an object of the invention, with a processfor manufacturing a dental restoration as described above beingpreferred wherein the hot pressing comprises subjecting the lithiumsilicate material to a heat treatment at a temperature of about 500 to1200° C. to convert the lithium silicate material into a viscous stateand pressing the viscous lithium silicate material under a pressure ofabout 1 to 4 MPa into a mould or dye to obtain the dental restorationwith a desired geometry.

A process as described above, wherein the lithium silicate materialsubjected to the heat treatment and pressing comprises lithiummetasilicate crystals which are converted into lithium disilicatecrystals during the heat treatment and pressing is more preferred.

A further preferred embodiment of the present invention is formed by aprocess as described above which comprises an increasing of strength andfracture toughness of the lithium silicate material.

A process for the manufacture of a dental restoration as described aboveis preferred, wherein the dental restoration has a biaxial strength ofat least 250 MPa and a fracture toughness of at least 1.5 MPam^(0.5).

A process for the manufacture of a dental restoration as described abovefurther comprising finishing the dental restoration to obtain a naturalappearance is preferred.

Same is true for a process as described above, wherein the finishingstep comprises applying a coating to the dental restoration by layeringwith powdered materials or by hot pressing a coating material onto theunfinished dental restoration.

A process as described above wherein the third heat treatment occursduring a firing of the layering materials or the hot pressing of thecoating material onto unfinished the dental restoration is even morepreferred.

Thus, a product is finally obtained which has all the beneficialmechanical, optical and stability properties making lithium disilicateceramics attractive for use as dental restorative materials. However,these properties are achieved without the disadvantages of theconventional materials when shaped by using a CAD/CAM based process, inparticular the undue wear of the milling and trimming tools.

Consequently, the invention also relates to a lithium disilicate glassceramic product which is obtainable by the above process for itspreparation and has lithium disilicate as main crystalline phase.Preferably, the lithium disilicate glass ceramic product according tothe invention is in the form of a dental restoration.

It is further preferred that in the lithium disilicate glass ceramic thelithium disilicate crystals form 60 to 80% by volume of the glassceramic.

The conversion of the lithium metasilicate glass ceramic according tothe invention to a lithium disilicate glass ceramic product isassociated with a surprisingly high increase in strength by a factor ofup to about 4. Typically, the lithium metasilicate glass ceramic of theinvention has a strength of about 100 MPa, and the conversion leads to alithium disilicate glass ceramic having a strength of more than 400 MPa(measured as biaxial strength).

The invention is also directed to a lithium silicate blank as describedabove, wherein the holder is jointed and connected with the holder.

A lithium silicate blank as described above, wherein the holder is froma different material from the blank forms one embodiment of theinvention.

A lithium silicate material blank as described above, wherein the holderis made from an alloy, from a metal, from a glass ceramic or from aceramic forms a preferred embodiment of the invention.

A lithium silicate blank as described above, wherein the holder is madefrom the same material as the blank and is integral with the blank isanother embodiment of the invention.

A lithium silicate blank as described above, wherein the blank islabeled with information is another preferred embodiment.

Same is true for a lithium silicate blank as described above, whereinthe information on the blank comprises the material, the size and thetype of the shape, which is to be machined from the blank.

The invention is also directed to a lithium silicate glass-ceramicdental product comprising lithium metasilicate and having a flexuralstrength of 180 MPa or greater, for example in the range of 180 to 200MPa to about 400 MPa or greater and a fracture toughness less than about2.0, preferably less than about 1.5.

A process described herein for manufacture of a dental article isanother object of the invention, wherein a lithium metasilicate block isfabricated having a strength of about 180 or 200 MPa to about 400 MPaand a fracture toughness less than about 2.0, preferably less than about1.5 and milling the block into a dental restoration. The dentalrestoration may be used as is or further heat treated and/or coated.

Another aspect of the present invention is directed to a method formanufacturing a lithium silicate restoration comprising preparing alithium silicate blank as described above, and thereafter coating adental restoration with the lithium silicate blank.

A method for manufacturing a dental restoration as described abovewherein a dental framework is coated by hot pressing the lithiumsilicate blank onto the dental framework is preferred.

A method for manufacturing a dental restoration as described above,wherein the dental framework is a crown, a partial crown, a bridge, acoping, a veneer, a facing or an abutment is more preferred and such amethod, wherein the dental framework is made from a metal, an alloy, aceramic or a glass ceramic is even more preferred.

A method for manufacturing a dental restoration as described above,wherein the ceramic comprises zirconium oxide, aluminium oxide, azirconium mix oxide, an aluminium mix oxide, or a combination thereofforms a particularly preferred embodiment of the invention.

A method for manufacturing a dental restoration as described abovewherein the lithium silicate blank which is coated onto the frameworkcomprises lithium metasilicate crystals which are converted to lithiumdisilicate crystals, or the lithium silicate blank comprises nucleisuitable for forming lithium metasilicate crystals which crystallize aslithium disilicate crystals during the hot pressing of the lithiumsilicate blank onto the dental framework is another preferred object ofthe invention.

The invention is explained in more detail below on the basis ofExamples.

EXAMPLES Examples 1 to 18 (Invention), 19 to 20 (Comparison) and 21 to23 (Invention) and 24 to 25 (Invention)

A total of 20 different lithium metasilicate glass ceramic productsaccording to the invention as well as two ceramics for comparison withthe chemical compositions given in Table III were prepared by carryingout stages (a) to (d) of the process described above and finallyconverted to lithium disilicate glass ceramic products by step (e) ofthe process described above:

For this purpose samples of the corresponding starting glasses weremelted in a platinum-rhodium crucible at a temperature of 1500° C. andfor a period of 3 hours (a).

The glass melts obtained were then poured into steel moulds which werepreheated to 300° C. After 1 minute the glass blanks were transferredinto a furnace which was preheated to a temperature between 450 and 550°C. The exact values, KB T [° C.] and KB t [min], are given for eachsample in Table III. After this relaxation and nucleation process (b andc) the blocks were allowed to cool to room temperature. The nucleatedsamples were homogeneous and transparent.

The glass blanks, which contained nuclei for the crystallization, werethen subjected to step (d), i.e. the second heat treatment, tocrystallize lithium metasilicate, which means that the glass blanks wereexposed to a temperature of about 650° C. for a period of about 20minutes, except example 3, which was crystallized at 600° C.

The course of the crystallization was investigated by DSC-measurementand the resulting crystal phases were analyzed by XRD to identify theideal conditions for this heat treatment. “Ideal conditions” in thesense of the present invention are present in case the twocrystallization peaks of the meta- and the disilicate phase respectivelyare differing to such an extend that in the production process a neatdifferentiation can be implemented, i.e. when heating a sample to thefirst crystallization temperature it has to be secured that whenreaching the desired temperature within the sample the temperature atthe outer regions of the sample does not reach the secondcrystallization temperature, i.e. the bigger the temperature differenceof the first and the second crystallization temperature is the biggerthe sample mass can be.

To further illustrate the process FIG. 2 shows a DSC-plot of one of theexamples, example 13, a quenched and powdered glass sample, which washeated with a heating rate of 10 K/min. The crystallisation of lithiummetasilicate (1), the crystallisation of lithium disilicate (2) as wellas the glass transition temperature (3) and the temperature range (4)for the first crystallisation are clearly visible from said DSC-plot.

Also an example for the analysis of phase development by hightemperature XRD from the same example 13 is given. FIG. 3 thereforeshows the measurement of a bulk glass sample at a constant heating rateof 2 K/min. It can be recognized from said measurement that in this casethe crystallisation of the lithium metasilicate (1) occurs at atemperature of 510° C. and that in this case the resolution of thelithium metasilicate and the crystallization of the lithium disilicate(2) occur at a temperature of 730° C.

FIG. 4 represents a phase analysis by XRD of example 13 after nucleationat 500° C. for 7 min and first crystallisation at 650° C. and 20 min.

The corresponding data are summarized In Table I:

TABLE 1 1 2 d-spacing in 0.1 d-spacing in 0.1 3 nm of scan nm of patternIndex 4.628 4.690 LS 020 3.296 3.301 LS 111 2.708 LS 130 2.685 2.700 LS200 2.355 2.342 LS 131 2.333 2.331 LS 002

FIG. 5 shows an SEM-micrograph, backscattered electrons, of the sameexample having the same thermal history, with the surface being etchedwith 1% HF for 8 s. Clearly visible are holes that show former lithiummetasilicate crystals.

The resulting blocks were now ready for step (e), which means shapingthe lithium metasilicate glass ceramic to the desired shape, either bysaw cutting, or by milling it in a CAD-CAM milling machine (i.e. CEREC3®). The obtained lithium metasilicate glass ceramic blanks wereanalyzed for their machinability and their edge strength. 10 discs werecut from a rod with 12 mm diameter for biaxial strength measurements.The results of these analyses are given in Table IV. Ten more discs wereprepared and subjected to a third heat treatment (f).

In case the blanks contain colouring and fluorescent oxides the blocksin the state of the metasilicate appear to have a reddish or bluishcolour. This effect vanishes when the disilicate phase forms and theblanks turn to the colour that is desired.

Finally, the lithium metasilicate glass ceramic blanks were subjected toa second crystallization, step (f), at 850° C. for 10 min, exceptexample 3 which was crystallized at 830° C., i.e. the third heattreatment which is in general performed at temperatures of 700 to 950°C., preferably 820 to 880° C. and for a period of 5 to 30 minutes,preferably 5 to 20 minutes, to convert the lithium metasilicate intolithium disilicate. Optionally, it is not necessary to further heattreat the lithium metasilicate to convert it to lithium disilicate. Itis possible to mill the dental article from the lithium metasilicateglass ceramic blanks with no further heat treatment.

The obtained products were analyzed for their crystal phases. To furtherillustrate the procedure the phase analysis for example 13 afternucleation at 500° C. for 7 min, first crystallization at 650° C. for 20min and second crystallization at 850° C. for 10 is shown in FIG. 6. Thecorresponding data are summarized in Table II.

TABLE II 1 2 d-spacing in 0.1 d-spacing in 0.1 3 nm of scan nm ofpattern Index 5.369 5.420 LS2 110 3.986 3.978 LP 120 3.855 3.834 LP 1013.714 3.737 LS2 130 3.629 3.655 LS2 040 3.562 3.581 LS2 111 2.929 2.930LS2 131 2.901 2.908 LS2 200 2.379 2.388 LS2 002 2.346 2.35 LS2 221 2.2832.29 LS2 151 2.050 2.054 LS2 241

FIG. 7 shows an SEM-micrograph, backscattered electrons, of the sameexample having the same thermal history, with the surface being etchedwith 3% HF for 30 s leading to the glassy phase being etched out andleaving the lithium disilicate crystals.

In addition to the analysis in respect to crystal phases the sampleswere also analyzed in respect to their biaxial strength and chemicaldurability. Furthermore, their translucence was assessed. The resultsare also given in Table IV.

In table IV, the detected crystalline phases are designated as follows:

-   -   LS—lithium metasilicate    -   LS2—lithium disilicate    -   LP—lithium phosphate,        with the main phase being marked in bold type.

To gain information about the machinability tests were performed on aCerec® 3, with new tools being used for each test. A ‘Lego®-Minicube’served as a model which had to be milled from all compositions that weresubjected to this test and from a leucite-enforced glass ceramic of thename ProCAD® from Ivoclar Vivadent AG. The operating sequence was asfollows: First a blank of ProCAD@ was milled, then a blank of theceramic to be tested was milled and after that again a ProCAD® blank wasmilled. The machinability was rendered “very good” in case the time thatwas required to mill the blank of the ceramic to be tested was below 95%of the time that was required to mill the ProCAD® blank. Times in therange of 95 to 105% of said time led to the mark “good” for themachinability, times in the range of 105 to 115% to “acceptable” andtimes above 115% to “poor”. The medium time required for the millingprocess was 14.0 minutes.

To compare the machinability of the test samples with another glassceramic a blank made according to the composition disclosed in DE 197 50794 was prepared and subjected to the test described above. After 15minutes the test was abandoned since only about 10% of the volume to bemilled was already milled and the tools used for milling were alreadyworn out, something that did not happen with any of the test samples.

The edge strength was determined as follows:

With a milling unit (CEREC 3®) blanks were milled to result inLego-minicubes. With a 1.6 mm cylindrical diamond cutter blind holeswere milled. The quality of said blind holes was determined by comparingthe area of the broken out edges with those of a reference sample(ProCAD®). The relation of the area of the broken out edges to the areaof the blind bore is an allocation for the edge strength.

An edge strength is considered to be “very good” in case the relation ofsaid areas is smaller than that of the reference, it is considered to be“good” in case the relations are about the same and it is considered tobe “acceptable” in case the area is bigger than 110% of the referencesample.

The chemical durability was determined according to ISO 6872, i.e. asloss of mass after 16 h in 4% acetic acid at 80° C.

“Good” means that the solubility according to said method is below 100pg/cm².

The strength was measured as biaxial strength according to ISO 6872 oras 3 point bending strength according to EN 843-1:

Bars of 12 mm diameter were casted and crystallized once. From thesebars 20 discs with a thickness of 1, 2 mm each were sawn. 10 of thesediscs were then smoothed and the surfaces of the discs were polishedusing SiC-paper of grain size 1000. Biaxial strength was measured as isdisclosed in ISO 6872. The other 10 discs were crystallized a secondtime at 800 to 900° C. to give the lithium disilicate phase. Thesesolidified samples were smoothed on both sides and the surfaces werepolished using SiC-paper of grain size 1000. Biaxial strength was thenmeasured according to ISO 6872.

By comparison bending strength was measured on bars with dimensions of25*3.5*3.0 mm were sawn out of a block of the lithium metasilicate glassceramic. These bars were smoothed to result in bars having dimensions of25*2.5*2.0 mm which were then polished using SiC-paper of grain size1000. The edges were also beveled with SiC-paper of grain size 1000. Thespan was 20 mm. The results are comparable to biaxial strength results.

In addition to this, fracture toughness was determined by applying aVickers indentation onto a polished surface and measuring the size ofthe flaws originating from the edges (Indentation Force Method . . .IF). This method is useful as comparative method but does not result inabsolute values. For comparison measurements were performed on notchedbending samples (SENB, SEVNB). For the lithium disilicate glass ceramicsfracture toughness values >2 MPam^(0.5) were obtained.

In Tables IV and V the values for the biaxial strength and the fracturetoughness of the samples having the disilicate phase, i.e. those samplesthat were crystallized twice, are given. In addition to that quotientsare given which give the ratio of the biaxial strength of the disilicatesystem to the biaxial strength of the metasilicate system (biaxialsolidification factor) or the ratio of the fracture toughness of thedisilicate system to the fracture toughness of the metasilicate system(solidification factor K1C).

Translucence was determined after the second crystallization: a testpiece 16 mm diameter and having a thickness of 2 mm was prepared andpolished on both sides. The contrast value CR was determined accordingto BS 5612 (British Standard) using a spectral colorimeter (MinoltaCM-3700d). The determination of the contrast value consisted of twosingle measurements. The test piece to be analyzed is therefor placed infront of a black ceramic body having a reflexion of 4% at most andaccordingly in front of a white ceramic body having a reflexion of 86%at minimum which are then colourmetrically determined. Using highlytransparent test pieces reflexion/absorption is mainly caused by theceramic background whereas reflexion is caused by the test piece in casean opaque material is used. The ratio of reflected light on black groundto reflected light on white ground is the quantum for the contrastvalue, with total translucence leading to a contrast value of 0 andtotal opaquescence leading to a contrast value of 1. The samples wererated as follows:

-   -   extraordinary: CR<0.4    -   very good: 0.4<CR<0.5    -   good: 0.5<CR<0.6    -   acceptable: 0.6<CR<0.8

opaque: 0.8<CR.

TABLE III Expl No. 1 2 3 4 5 6 7 8 9 10 KBT [° C.] 500 490 520 500 500500 500 500 500 500 KBt [min] 10 30 5 30 10 10 10 10 10 10 wt % SiO₂69.3 73.0 64.0 68.1 70.1 69.0 68.6 69.9 68.6 68.8 K₂O 4.3 4.4 4.2 4.24.5 4.3 4.3 4.4 2.0 5.0 Na₂O 2.0 SrO 2.0 BaO 2.0 2.0 CaO 2.0 Li₂O 15.317.0 13.0 15.0 15.5 15.2 15.1 15.4 15.1 15.1 Al₂O₃ 1.1 1.1 4.0 5.0 1.11.1 1.1 1.1 3.0 1.1 P₂O₅ 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 5.0 3.8 MgO 1.00.0 1.0 0.0 5.0 1.0 1.0 1.0 0.0 1.0 ZrO₂ 2.o ZnO 5.2 0.7 6.0 3.9 0.0 3.64.1 2.4 4.3 3.2 TiO₂ V₂O₅ Fe₂O₃ MnO₂ CeO₂ 2.0 Y₂O₃ La₂O₃ Pr₂O₃ Ta₂O₅Tb₄O₇ Er₂O₃ 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0Expl No. 11 12 13 14 15 16 17 18 19 20 KBT [° C.] 520 500 500 500 500500 500 500 500 500 KBt [min] 10 10 7 7 7 10 20 10 10 30 wt % SiO₂ 70.065.7 67.4 68.4 65.0 70.0 70.0 67.8 68.3 67.7 K₂O 5.0 4.1 4.0 2.7 2.0 4.43.8 4.1 4.3 4.2 Na₂O 0.1 1.0 0.1 0.1 0.1 0.1 SrO 2.0 BaO 2.0 CaO 1.0Li₂O 15.0 14.5 14.8 15.0 14.0 16.0 16.0 15.0 15.1 14.9 Al₂O₃ 1.1 1.1 1.13.0 4.1 1.8 1.1 1.1 0.0 0.0 P₂O₅ 2.0 3.8 3.8 3.5 3.8 3.8 3.8 3.8 3.8 3.8MgO 0.9 1.0 0.5 0.1 0.0 0.3 0.1 0.1 1.0 1.0 ZrO₂ 1.0 0.1 0.1 0.1 0.1 0.10.1 ZnO 6.0 2.8 4.7 5.2 4.0 2.0 4.5 4.8 5.1 5.0 TiO₂ 1.6 V₂O₅ 0.2 Fe₂O₃0.2 MnO₂ 0.2 0.5 CeO₂ 0.5 2.0 1.0 0.4 1.0 0.4 0.5 Y₂O₃ 2.4 La₂O₃ 0.5 0.31.0 0.1 0.1 0.3 3.4 Pr₂O₃ 1.0 Ta₂O₅ 1.5 Tb₄O₇ 1.5 0.5 0.5 0.5 Er₂O₃ 1.00.3 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

TABLE IV Ex. No. 1 2 3 4 5 6 7 8 9 10 phases present after LS LS, LS2 LSLS LS, LS2 LS LS LS, LS2 LS LS 1st crystallisation phases present afterLS2, LP LS2, LP LS2, LP LS2, LP LS2, LP lS2, LP LS2, LP LS2, LP LS2, LPLS2, LP 2nd crystallisation biaxial strength after 359 424 250 314 324472 426 404 356 319 2nd crystallisation biaxial solidification 3.0 2.42.5 3.4 2.4 3.5 3.5 2.3 3.2 2.7 factor K1C [MPm^(0.5)] after 1.6 2.2 1.91.9 1.8 2.3 1.8 2.4 1.9 1.8 2nd crystallisation K1C solidification 1.81.7 2.6 2.5 1.6 2.4 2.0 1.9 1.9 1.8 factor grinding time in 93% 103% 95%89% 98% 93% 94% 105% 94% 94% comparison to ProCAD machinability verygood good very good very good good very good very good good very goodvery good edge strength good very good good good good good acceptablegood acceptable good translucency very good n.m. n.rn. n.m. n.m.extraordinary n.m. n.m. acceptable n.m. chemical durability good goodgood good good good good good good good (ISO 6872) Ex. No. 11 12 13 1415 16 17 18 19 20 phases present after LS, LS2 LS LS LS LS LS LS LS LS2LS2 1st crystallisation phases present after LS2, LP LS2, LP LS2, LPLS2, LP LS2, LP LS2, LP LS2, LP LS2, LP LS2, LP LS2, LP 2ndcrystallisation biaxial strength after 301 354 381 389 342 329 420 387440 405 2nd crystallisation biaxial solidification 1.7 2.9 3.0 3.1 2.62.9 3.2 3.4 2.2 2.1 factor K1C [MPm^(0.5)] after 2.1 2.0 1.9 2.0 1.8 1.72.1 1.9 1.8 1.9 2nd crystallisation K1C solidification 1.6 1.9 2.1 1.82.0 1.6 2.2 1.9 1.0 1.5 factor grinding time in 115% 98% 90% 91% 94%100% 95% 94% 119% 129% comparison to ProCAD machinability acceptablevery good very good very good very good good good very good poor pooredge strength good acceptable very good very good very good very goodvery good good good good translucency n.m. n.m. n.m. n.m. n,m. n.m, n,m.n.m. n.m. n.m. chemical durability good good good good good good goodgood good good (ISO 6872)

Table V shows the flexural strength and the fracture toughness for themachinable condition and the heat treated condition. Although the higherstrengths of 200 and 193 of the metasilicate were shown having poormachinability, it has been found that such strengths and higher, incombination with low fracture toughness prove to have goodmachinability, especially for use with ultrasonic type machining andwhen using fluted tools.

TABLE V Flexural Strength, MPa Fracture Toughness (KIc), MPam^(0.5)Machinable Machinability Machinable Edge Example Final¹ Condition² SF³by Cerec⁷ Final⁴ Condition⁵ KIcF⁶ Strength 1 359 120 3.0 very good 1.60.9 1.8 good 2 424 177 2.4 good 2.2 1.3 1.7 very good 3 250 100 2.5 verygood 1.9 0.7 2.6 good 4 314 92 3.4 very good 1.9 0.8 2.5 good 5 324 1352.4 good 1.8 1.1 1.6 good 6 472 135 3.5 very good 2.3 1.0 2.4 good 7 426122 3.5 very good 1.8 0.9 2.0 acceptable 8 404 176 2.3 good 2.4 1.3 1.9good 9 356 111 3.2 very good 1.9 1.0 1.9 acceptable 10 319 118 2.7 verygood 1.8 1.0 1.8 good 11 301 177 1.7 acceptable 2.1 1.3 1.6 good 12 354122 2.9 very good 2 1.1 1.9 acceptable 13 381 127 3.0 very good 1.9 0.92.1 very good 14 389 125 3.1 very good 2 1.1 1.8 very good 15 342 1322.6 very good 1.8 0.9 2.0 very good 16 329 113 2.9 good 1.7 1.1 1.6 verygood 17 420 131 3.2 good 2.1 1.0 2.2 very good 18 387 114 3.4 very good1.9 1.0 1.9 good 19 440 200 2.2 poor 1.8 1.8 1.0 good 20 405 193 2.1Poor 1.9 1.3 1.5 good ¹Final is the strength of the disilicate system.²Machinable Condition is the strength of the metasilicate system. ³SF isthe biaxial strength solidification factor. ⁴Final is the fracturetoughness of the disilicate system. ⁵Machinable Condition is thefracture toughness of the metasilicate system. ⁶KIcF is the fracturetoughness solidification factor. ⁷Machinability given in this table isonly for Cerec 3 (Sirona) and may be even better for larger CAD/DAMsystem.

The data in Table II show that the lithium metasilicate glass ceramicscombine a very good machinability and high edge strength with the easypossibility to convert them by a simple heat treatment into lithiumdisilicate glass ceramics which have a very high bending strength aswell as an excellent chemical durability and good translucence, all ofwhich being properties which make them very attractive as materialsuseful for the manufacture of dental restorations.

In the following some examples are described in more detail:

Example 1

The glass was molten at a temperature of 1500° C. for 3 hours and wasthen poured into steel moulds which were preheated to 300° C. After oneminute the glass bars were transferred into a cooling furnace and weretempered at 500° C. for 10 minutes and then cooled to room temperature.

The glass was homogeneous and transparent.

Following the glass bar was subjected to a first crystallization at 650°C. for a period of 20 minutes.

From the such ceramized bar, discs were sawn out of a round bar, andbiaxial strength was measured. The phase content was analyzed via XRD(X-ray diffraction). Lithium metasilicate was the only phase that wasdetected. Biaxial strength was 119+/−25 MPa.

Also the milling time of test bodys was measured. The milling time ofthe test body was one minute below that of ProCAD®, which was used asreference.

The edge strength was good.

Additional 10 discs were subjected to a second crystallization at 850°C. for a period of 10 minutes and biaxial strength and fracturetoughness were measured.

Biaxial strength was 359+/−117 MPa which correlates to a solidificationfactor of 3.0.

Fracture toughness (IF) was 1.6 MPam^(0.5).

Translucence was very good.

The chemical stability according to ISO 6872 (4% acetic acid, 80° C., 16h) was 37 pg/cm².

Example 6

Glass bars were produced according to example 1. The glass again washomogeneous and transparent.

The first crystallization was performed at 650° C. for a period of 20minutes.

Lithium metasilicate was determined to be the main phase with traces oflithium disilicate also being present. Biaxial strength was 135+/−24MPa.

Again the milling time of a test body was measured. The milling time ofthe test body was one minute below that of ProCAD®, which again was usedas reference.

The edge strength was very good.

After a second crystallization which was performed according to example1 the biaxial strength was 472+/−85 MPa which correlates to asolidification factor of 3.5.

Fracture toughness (IF) was 2.3 MPam^(0.5).

Translucence was extraordinary.

Example 9

Glass bars were produced according to example 1. The glass again washomogeneous and transparent.

The first crystallization was performed at 650° C. for a period of 20minutes.

Lithium metasilicate was determined to be the only phase. Biaxialstrength was 112+/−13 MPa.

Again the milling time of a test body was measured. The milling time ofthe test body was one minute below that of ProCAD®, which again was usedas reference.

The edge strength was good.

After a second crystallization which was performed according to example1 the biaxial strength was 356+/−96 MPa which correlates to asolidification factor of 3.16.

Fracture toughness (I F) was 1.9 MPam^(0.5).

Translucence was acceptable.

Example 20 (Comparison)

Glass bars were produced according to example 1. The glass again washomogeneous and transparent.

The first crystallization was performed at 650° C. for a period of 20minutes.

Lithium disilicate was determined as the main phase and lithiummetasilicate was only present in traces. Biaxial strength was 194+/−35MPa.

Again the milling time of a test body was measured. The milling time ofthe test body was four minutes longer that of ProCAD®, which again wasused as reference.

The edge strength was poor.

After a second crystallization which was performed according to example1 the biaxial strength was 405+/−80 MPa which correlates to asolidification factor of 2.09.

Fracture toughness (IF) was 1.88 MPam^(0.5).

Translucence was very good.

This example makes it even more obvious that in the light of the glassceramic materials according to the invention the adverse properties inrespect to machinability of the prior art material disqualify same to beused in applications as are mentioned above.

The following examples 21 to 23 show the usefulness of the lithiumsilicate glass according to the invention which comprises nucleisuitable for the formation of lithium metasilicate and subsequentlylithium disilicate glass ceramics.

Example 21

A glass melt having the composition according to example 14 was meltedin a platinum crucible at a temperature of 1500° C. for 3 hours. Theglass was not poured in a steel mould, but quenched in water.

Thus, a glass granulate formed which was dryed and subsequently heatedto 500° C. for 30 minutes to produce nuclei suitable for the formationof lithium metasilicate crystals. The obtained glass was milled to aparticle size of less than 45 pm.

The obtained powder was mixed with a modeling liquid consisting of morethan 95% water and additives for improving moldability and layered on acrown cap of densely sintered zirconium oxide, e.g. DCS-zircon.

The crown cap was fired in a dental furnace at a temperature of 900° C.with 2 minutes holding time. By this procedure the applied glass powdercontaining nuclei for the crystallization was simultaneouslycrystallized and densely sintered so that a dentine core of lithiumdisilicate glass ceramic resulted. On this core a suitable incisal masshaving a suitable expansion coefficient was applied.

The final anterior tooth restauration showed good resistance againstrapid temperature changes up to 160° C. This proves a good bond betweenthe dentine layer of lithium disilicate glass ceramic and the frameworkof high-strength zirconium oxide.

Example 22

Bars of a glass having a composition according to example 14 wereprepared in the same manner as in example 1. The glass bars werehomogenous, transparent and light yellow coloured. A crown cap ofdensely-sintered zirconium oxide was circularly reduced. Subsequently adentine core was layered with dental wax and the crown margin wasmodeled on the stump. The restauration was provided with a cast-onchannel. The crown cap was applied on a muffle basis and embedded ininvestment material, (Empress Speed, Ivoclar). After the requiredbinding time the muffle was preheated to 850° C. resulting in theremoval of the wax. After 90 minutes a blank of the above lithiumsilicate glass having nuclei for forming lithium metasilicate was put inthe muffle and pressed on the cap of zirconium oxide in accordance withthe known Empress-hot pressing process at 900° C. This resulted incrystallization of the glass blank to a lithium disilicate glassceramic.

After divesting, the final product was a zirconium oxide cap having adentine layer of lithium disilicate glass ceramic. This dentalrestoration showed an excellent fit of the circular edge on the model.Furthermore, the so-prepared dentine layer was free from porosities.

Example 23

A metal cap of an alloy having an expansion coefficient of 12.8*10⁻⁶ 1/Kand a solidification temperature of 11000° C. was prepared in a castprocess, sand-blasted and by an oxidation-firing step prepared for thefurther processing.

In an analogous manner as in example 22 a dentine core was applied onthe cap using modeling wax. The metal cap was embedded, and the wax wasremoved by firing in a furnace. As in example 22 a blank of the lithiumsilicate glass having suitable nuclei was hot-pressed on the metal capat 900° C.

The so-prepared dental restoration showed a good bond between metalframework and the lithium disilicate glass ceramic and also had a highresistance against drastic temperature changes of above 160° C.

TABLE VI Example 24 Example 25 SiO₂ 70.9 wt.-% 70.4 wt.-% Li₂O 14.7 14.6  K₂O 4.7 4.0 P₂O₅ 3.3 3.2 Al₂O₃ 1.4 3.4 ZrO₂ 1.0 0.8 CeO₂ 2.0 2.0ZnO 0.6 — Er₂O₃ 0.6 0.6 Tb₄O₇ 0.5 0.5 V₂O₅ 0.3 0.3 MgO — 0.3 Melting1500° C./2 h   1500° C./2 h   Nucleation 500° C./10 min 500° C./10 minCrystallization 660° C./10 min 670° C./10 min Biaxial flex. 297 ± 39 247 ± 42 Strength [MPa] microstructure Lithium metasilicate Lithiummetasilicate Fracture toughness 1.20 ± 0.12 — [MPam^(0.5)]

Examples 24 and 25 show results for lithium metasilicate having highflexural strength (in the range of about 300 MPa) in combination with alow fracture toughness of 1.0 to 1.4 MPam^(0.5). It has been realizedthat, due to the relatively low fracture toughness and hardness, themetasilicate blocks having high strengths can be machined within goodtime ranges per unit, using CAD/CAM systems such as DCS Precimill (DCSSwitzerland) or Roeders 5-axis CNC machines (ROEDERS GmbH, Germany). Themilling times range from less than about 45 minutes per unit, preferably30 minutes or less and most preferably 15 minutes or less per unit,wherein unit is crown, partial crown, pontic, abutment, inlay or onlay.

Examples 26 to 33

A total of 8 different lithium metasilicate glass ceramics according tothe invention with the chemical compositions given in Table VII wereprepared using the indicated second heat treatment. The obtained glassceramics were then converted to the corresponding lithium disilicateglass ceramics using the indicated third heat treatment.

Firstly, samples of the corresponding starting glasses were melted in aplatinum-rhodium crucible at a temperature of 1450° C. and for a periodof 40 minutes. The glass melt was poured into water and the obtainedgranules were, after drying, again melted at 1500° C. The glass meltsobtained were then poured into graphite moulds to give blocks. Afterrelaxation of the glass blocks at 500 to 600° C. for 10 minutes to 3hours, they were subjected to the given second heat treatment. Beforeeffecting the third heat treatment, the blocks were checked for theirmachinability by milling in a CAD-CAM milling machine (i.e. CEREC 3®).Finally, the indicated third heat treatment was conducted. The crystalphases present after the second and third heat treatment were identifiedby XRD techniques and are given in table VII.

Further, the opalescence of the products was visually assessed and thecontrast value CR was determined according to BS 5612 (British Standard)using a spectral colorimeter (Minolta CM-3700d). The chemical stabilityin acetic acid was determined as well as the stability in artificialsaliva. The corresponding data are to be found in the following TableVIII and show in particular the surprising combination of a lack ofopalescence together with a high translucence and stability. Thecomposition of the artificial saliva is given in Table IX.

The data obtained show that the lithium metasilicate glass ceramicsaccording to the invention combine a very good machinability and highedge strength with the easy possibility to convert them by a simple heattreatment into lithium disilicate glass ceramics which have a very highbending strength as well as an excellent chemical durability and goodtranslucence, all of which being properties which make them veryattractive as materials useful for the manufacture of dentalrestorations.

Examples 34 to 37

Four glass ceramics according to the invention were prepared inanalogous manner as examples 26 to 33. However, the heat treatmentscheme was different. In addition each material was subjected to theschemes referred to as “Cycle A” and “Cycle B” which differ in thetemperature used for the crystallization of lithium metasilicate, namely650° and 700° C., respectively. Strengths were measured for both CyclesA and B after crystallization of lithium metasilicate (heat treatment 2)and after crystallization of lithium disilicate (heat treatment 3).

Details as to the materials prepared and tested as well as theirproperties are given in the table X. It is apparent that the “Cycle B”treatment using a temperature of 700° C. for the crystallization oflithium metasilicate leads to lithium disilicate glass ceramics havingexcellent strengths.

TABLE VII Example 26 27 28 29 30 31 32 33 Molar ratio SiO2:Li2O 2.39:12.39:1 2.4:1 2.39:1 2.39:1 2.39:1 2.39:1 2.39:1 Al2O3:K2O   1:1.0  1:1.0   1:1.2    1:1.20    1:1.35    1:1.50    1:1.70    1:1.30 wt.-%wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% (Mol %) (Mol %) (Mol %) (Mol%) (Mol %) (Mol %) (Mol %) (Mol %) SiO2 72.21 (66.12) 70.64 (65.62)70.52 (65.52) 70.78 (65.57) 70.78 (65.56) 70.78 (65.56) 70.78 (65.55)70.78 (65.29) K2O 3.16 (1.85) 3.09 (1.83) 3.81 (2.26) 3.76 (2.22) 3.96(2.34) 4.16 (2.46) 4.36 (2.58) 3.36 (1.98) Li2O 14.99 (27.60) 14.68(27.43) 14.64 (27.35)  14.7 (27.38)  14.7 (27.38)  14.7 (27.37)  14.7(27.37)  14.7 (27.26) Al2O3 3.45 (1.86) 3.38 (1.85) 3.35 (1.83) 3.38(1.85) 3.18 (1.74) 2.98 (1.63) 2.78 (1.52) 2.78 (1.51) P2O5 3.28 (1.27)3.21 (1.26)  3.2 (1.26) 3.21 (1.26) 3.21 (1.26) 3.21 (1.26) 3.21 (1.26)3.21 (1.25) ZrO2 2.91 (1.30) 3.00 (1.36)  2.5 (1.13)  1.8 (0.81)  1.8(0.81) 1.80 (0.81)  1.8 (0.81)  1.8 (0.81) CeO2 1.88 (0.61) 1.86 (0.60)2.00 (0.65) 2.00 (0.65) 2.00 (0.65) 2.00 (0.65) 2.00 (0.65) V2O5 0.12(0.04) 0.12 (0.04)  0.07 ((0.02)  0.07 ((0.02)  0.07 ((0.02) 0.07 (0.02)0.07 (0.02) MnO2 0.03 (0.02) 0.03 (0.02) 0.03 (0.02) 0.03 (0.02) 0.03(0.02) Er2O3  0.12 (0.017)  0.12 (0.017)  0.12 (0.017)  0.12 (0.017) 0.12 (0.017) MgO 0.15 (0.21) 0.15 (0.21) 0.15 (0.21) 0.15 (0.21) 0.15(0.21) CaO 1.00 (0.99) Crystalline phases after: Second heat Li2SiO3Li2SiO3 Li2SiO3 Li2SiO3 Li2SiO3 Li2SiO3 Li2SiO3 treatment: Li2Si2O5 *Li2Si2O5 * Li2Si2O5 * 20′/650° C. Third heat Li2Si2O5 Li2Si2O5 Li2Si2O5Li2Si2O5 Li2Si2O5 Li2Si2O5 Li2Si2O5 Li2Si2O5 treatment: Li3PO4* Li3PO4*Li3PO4* Li3PO4* Li3PO4* Li3PO4* Li3PO4* Li3PO4* 10′/850° C.

TABLE VIII Example 26 27 28 29 31 CR-Value BS-5612 40.4 37.0 50.0 59.358.8 (1978) Opalescence No No No No No Chemical stability 9 18 48 3 9 inAcetic acid (24 h/80° C., mass loss in μg/cm²) Chemical stability 13 1728 27 17 in Saliva (7 d/60° C., mass loss in μg/cm²)

TABLE IX Composition of artificial saliva Component Amount in mg in atotal of 500 ml H₂O NaCl 125.64 KCl 963.9 NH₄Cl 178.0 CaCl₂•2H₂O 227.8KSCN 189.2 CO(NH₂)₂ 200.0 Na₂SO₄•10H₂O 336.2 NaHCO₃ 630.8 KH₂PO₄ 654.5

TABLE X Example 34 35 36 37 SiO₂ 74.37  72.89  72.21  71.40  K₂O 3.263.18 3.16 3.13 Li₂O 15.44  15.13  14.99  14.79  Al₂O₃ 3.55 3.48 3.453.41 P₂O₅ 3.38 3.31 3.28 3.22 ZrO₂ 0.00 2.01 2.91 4.05 All values abovein wt.-% SiO₂:Li₂O 2.39 2.40 2.39 2.40 (Mol.-ratio %) Cycle A: (1) 500°C./10 min + (2) 650° C./20 min + (3) 850° C./10 min *) Biaxial Flexural260 +/− 38 284 +/− 35 270 +/− 20 240 +/− 13 Strength/MPa after heattreatment(2) Biaxial Flexural 786 +/− 92 515 +/− 54 522 +/− 82 479 +/−36 Strength/MPa after heat treatment(3) Contrast Ratio 0.80 0.56 0.430.36 Cycle B: (1) 500° C./10 min + (2) 700° C./20 min + (3) 850° C./10min *) Biaxial Flexural 270 +/− 36 290 +/− 59 235 +/− 40 232 +/− 44Strength/MPa after heat treatment(2) Biaxial Flexural  828 +/− 104 659+/− 75 608 +/− 90  694 +/− 113 Strength/MPa after heat treatment(3)Contrast Ratio 0.83 0.63 0.53 0.41 *) (1) Nucleation in the glass (2)Crystallization of Li-Metasilicate (3) Crystallization of Li-Disilicatefrom Li-Metasilicate

Tool wear is another very important factor as diamond or diamond coatedtools are very expensive. Faster milling is normally associated withincreased tool wear. The milling times provided above are based on toollife span of at least ten units per conventional diamond tool such asdiamond plated burr of any shape and size. This will translate into atleast 5 hours of milling time or preferably 10 hours for a newconventional tool before it needs to be replaced. There are technologiesand milling tools available that are synergetic with lithium silicateglass-ceramic materials of the present invention such as ultrasonicallyaided machining and also use of fluted diamond coated tools such asend-mills and microend-mills coated with various continuous diamondcoatings or other hard coatings, both amorphous and crystalline such asdifferent types of CVD, PVD or plasma coatings.

Flexural or biaxial strength is measured in accordance with ISO6872:2008, which is included by reference herein. Flexural strengthunder ISO 6872:2008 is comparable to biaxial strength, three-point bendstrength and four-point bend strength. Milling, grinding, machining,cutting are used interchangeably to denote subtractive shaping processes

Singular form of terms may also include plural forms of the same termsuch as, for example, wherein machinable phase also includes acombination of various phases, such phase assemblage also beingmachinable.

Although the present invention has been described in connection withpreferred embodiments thereof, it will be appreciated by those skilledin the art that additions, deletions, modifications, and substitutionsnot specifically described may be made without department from thespirit and scope of the invention as defined in the appended claims.

1. A lithium silicate dental glass-ceramic article for producing adental restoration comprising: metasilicate as the main crystallinephase with 20-50 vol % lithium metasilicate crystalline phase.
 2. Thelithium silicate dental glass-ceramic article of claim 1, wherein thearticle is a blank that is fixed to a holder.
 3. The lithium silicatedental glass-ceramic article of claim 2, wherein the blank is millableto a dental restoration having a biaxial flexural strength from 250 to472 MPa measured according to ISO 6872 and exhibiting a final lithiumdisilicate phase.
 4. A dental restoration milled from the lithiumsilicate dental glass-ceramic article of claim 3, wherein the dentalrestoration is an inlay, an onlay, a bridge, an abutment, a facing, aveneer, a facet, a crown, a partial crown, a framework, or a coping. 5.A lithium silicate dental glass-ceramic article for producing a dentalrestoration comprising: 20-50 vol % lithium metasilicate crystallinephase, exhibiting edge strength with a fracture toughness of at least0.8 MPam^(0.5) and a biaxial flexural strength of at least 90 MPameasured according to ISO
 6872. 6. The lithium silicate dentalglass-ceramic article of claim 5, wherein the article is a blank that isfixed is fixed to a holder.
 7. The lithium silicate dental glass-ceramicarticle of claim 6, wherein the blank is millable to a dentalrestoration having a biaxial flexural strength between 250-472 MPameasured according to ISO 6872 and exhibiting a final lithium disilicatephase.
 8. A dental restoration milled from the lithium silicate dentalglass-ceramic article of claim 7, wherein the dental restoration is aninlay, an onlay, a bridge, an abutment, a facing, a veneer, a facet, acrown, a partial crown, a framework, or a coping.
 9. A lithium silicatedental glass-ceramic article for producing a dental restorationcomprising: an initial phase comprising metasilicate as the maincrystalline phase with 20-50 vol % lithium metasilicate crystallinephase and exhibiting a biaxial flexural strength of at least 90 MPameasured according to ISO 6872, the initial phase is millable to adental restoration having a biaxial flexural strength between 250-472MPa measured according to ISO 6872 and exhibiting a final lithiumdisilicate phase.
 10. The lithium silicate dental glass-ceramic articleof claim 9, wherein the article is a blank that is fixed to a holder.11. A dental restoration milled from the lithium silicate dentalglass-ceramic article of claim 10, wherein the blank is milled andsintered into an inlay, an onlay, a bridge, an abutment, a facing, aveneer, a facet, a crown, a partial crown, a framework, or a coping.